NEUROHORMONAL MECHANISMS IN EMOTIONAL INHIBITION OF MILK EJECTION

Abstract

SUMMARY 1. Fifteen lactating rabbits were maintained on a regime of one daily nursing in which litter weights, milk yields and duration of nursing were recorded. 2. Sodium pentobarbitone anaesthesia blocked the milk-ejection reflex and prevented the removal of more than 15% of the full milk yield by the young. Intravenous injection of 50 mU oxytocin regularly restored normal milk removal; 10 and 20 mU did not. 3. In thirty-five out of forty-two experiments in which the does were suckled while under forcible restraint the amount of milk removed was reduced by 20–100%. In twenty-nine cases injection of 50 mU oxytocin restored normal milk removal. In the remaining six experiments this replacement therapy was fully effective only after the does had been anaesthetized. 4. Kymograph records of milk-ejection responses showed that normal milk removal was associated with a reflex milk-ejection response similar to that produced by 50 mU oxytocin, and incomplete milk removal with a reduced (=5 mU oxytocin) or absent milk-ejection response. Where injection of 50 mU oxytocin failed to restore normal milk removal in the conscious animal, the resulting milk-ejection response was reduced by an amount similar to that produced by injection of 1μg adrenaline. 5. The results indicate that, while activation of the sympathetico-adrenal system does occur, the main factor in emotional disturbance of the milk-ejection reflex is a partial or complete inhibition of oxytocin release from the posterior pituitary gland.